The invention relates to an overlay target design method for semiconductor fabrication to minimize the impact of lens aberrations on target projection.
Typically semiconductor devices are fabricated by optical lithography techniques using a projection imaging systems. A typical projection image system 20 is illustrated in FIG. 1. The system 20 at a minimum includes an illumination controller 22 and an illumination source 24 coupled with and controlled by controller 22. Illumination source 24 may include, for example, a mirror, a lamp, a light filter, and a condenser lens system. As used herein, the term xe2x80x9clightxe2x80x9d refers to light used in photolithography. The term xe2x80x9clightxe2x80x9d need not be restricted to visible light, but may also include other forms of radiation and lithography. For example, energy supplied by lasers, photons, ion beams, electron beams, or X-rays are included within the term xe2x80x9clightxe2x80x9d.
Illumination source 24 emits light or radiation that can pass through openings in mask 26. System 20 shows mask 26 positioned adjacent to light source 24; optionally, other devices such as one or more optical lenses could separate light source 24 and mask 26. The term xe2x80x9cmaskxe2x80x9d is not limited to a physical structure, but also includes a digitized image used in, for example, electron beam and ion beam lithography systems. For example, mask 26 may include a pattern for projecting a wiring or feature pattern of an integrated circuit. The pattern of mask 26 may include various image structures, for example, clear areas, opaque areas, phase shifting areas, and overlay targets. Mask 26 generally is a combination of clear areas and opaque areas, where the clear areas allow light from light source 24 to pass through mask 26 to form the mask""s image. Light passing through mask 26 is further transmitted by projection lens 30, which may be, for example, a reduction lens or a combination of lenses for focusing the mask pattern onto a projection surface 111, such as a semiconductor wafer covered with a photoresist layer. Typical semiconductor fabrication involves a four to ten times reduction of mask size 26 by projection lens 30. Projection surface 111 is held in position by a holding device (not shown), which may be part of or controlled by a stepper (not shown). Also shown in FIG. 1 is lens pupil 28 of projection imaging lens 30, which defines the numerical aperture of lens 30.
As the dimension of features on integrated circuits continue to decrease, the resolution limits of optical lithography are quickly being reached. One limit is caused by lens aberration, which is the failure of a lens, such as projection lens 30, to produce exact point-to-point correspondence between a received image, such as from mask 26, and a projection surface 111, such as a semiconductor die 100 a portion of which is illustrated in FIG. 2. One of the many types of lens aberrations in semiconductor device fabrication is coma aberrations which are optical aberrations that cause the image of a mask 26 to appear comet-shaped or blurred on die surface 113 (FIG. 2). Coma aberrations result in not only line width variations and/or pattern asymmetry, but also affect the location or placement of the mask image on the die surface 113. As discussed in Takashi Saito and Hisashi Watanabe""s article xe2x80x9cInvestigation of New Overlay Measurement Marks for Optical Lithography,xe2x80x9d J. Vac Sci Technol. B 16(6), November/December 1998, pp. 3415, 3418, during sub-micron device fabrication using optical lithography lens aberrations cause different displacement errors for overlay targets and device patterns.
A target is a feature on a mask 26, usually at the perimeter of the mask 26, that is transferred to die surface 113 during the illumination phase. The target helps to determine if the image transfer from mask 26 to die surface 113 was properly aligned relative to lower layers. Typically, the quality of the lithographic image alignment is measured by determining the alignment of a target on a lower level to a target on an upper or overlay level. In general the image transfer is successful if the target of the upper-layer is approximately centered with the lower-level target. An overlay measurement system is used to measure the distances and spaces between edges or boundaries of the upper and lower targets. It is critically important that the circuit pattern on one layer is accurately aligned with that of earlier layers. To evaluate the alignment of two layers, a target is formed on each layer. FIG. 2 is an illustration of a single integrated circuit (IC) die 100 fabricated on a semiconductor wafer. The locations of the electrical circuit or pattern and targets are represented by large box 120, hereinafter referred to as pattern, and small boxes 110, hereinafter referred to as targets, respectively. Typical dimensions for die 100 are 5 millimeters by 5 millimeters and typical dimensions for targets 110 are 10 microns by 10 microns.
A prior art method of mask alignment measurement will be described with respect to FIGS. 3 and 4. FIG. 3 is a top view of a prior art box-in-box target 110 of FIG. 2. FIG. 4 is a cross-sectional view of FIG. 3 along line IIIxe2x80x94III. The accuracy of the transfer of the targets 110 approximates the accuracy of the pattern 120 transfer. A first target 112 with a box pattern can be formed on surface 113 in a first layer or under layer 115 using well known lithography techniques. Typically, a silicon oxide material is deposited over first layer 115 to form a second layer 116. The second target 114 is formed in second layer 116 with dimensions smaller than target 112 using well known lithography techniques. The perimeter 117 of first target 112 and perimeter 118 of second target 114 can be viewed by optical measurement equipment. By measuring the distance between the two perimeters or isolated edges 117, 118 at several locations, the center positions of targets 112, 114 can be determined and positional deviations between the two targets 112, 114 can be determined. The overlay measurement provides a comparison of the alignment of the underlay target 112 of layer 115 with that of overlay target 114 of layer 116.
In the article by Saito and Watanabe the benefits of using fine pattern targets made up of thin lines instead of large box shaped patterns is discussed. Fine patterns targets, such as targets formed with thin line widths, are generally much closer to the actual circuit features dimensions than conventional large box patterns. Since lens aberrations typically induce line width variations and create alignment errors, using fine pattern targets allows more accurate detection of lens aberrations and alignment errors. In other words, the use of the typical box-in-box method (FIGS. 3-4) to determine mask 26 displacement errors is not very accurate for small device patterns, such as a quarter micron device fabrication (0.25 micron device feature size). Using targets with feature dimensions (size and pitch) similar to those of the circuit improves the detection of displacement errors.
For example, FIG. 5 is an illustration of a conventional fine pattern target system 200. Fine pattern targets 210, 220 are formed in a first layer 215 (FIG. 6) over surface 213. Fine pattern targets 230, 240 are formed in a second layer 216 (FIG. 6) over first layer 215. FIG. 6 is a cross-sectional view of FIG. 5 along line VIxe2x80x94VI. In known target systems such as target system 200 (FIG. 5), the first layer targets 210, 220 and second layer targets 230, 240 generally have the same pitch (P1). The term xe2x80x9cpitchxe2x80x9d refers to the distance between the outside edge of a first target and the outside edge of a second target. For example in FIGS. 5-6, the pitch of targets 210 and 220 are the distance between the perimeter 211 of target 210 and the perimeter 212 of target 220. In known target systems 200 the pitches for targets in layers 215, 216 are generally the same. In addition, target line widths (W1) are generally the same for targets in layers 215, 216. However even for targets in two different layers 215, 216 with the same line width W1 and pitch P1, changes in the illumination settings of light source 24, such as wave length, intensity, and annular size, used to form the targets 210, 220, 230, 240 can cause misplacement of the second layer targets 230, 240 due to projection lens 30 aberrations.
Illumination settings are an important design factor for optimizing circuit feature dimensions, as the settings often change from one layer to another. For example, if the second layer 216 targets 230, 240 of FIGS. 5-6 are formed using different illumination settings than that used for first layer 215 targets 210, 220 the light will be diffracted differently by the mask 26. Since the target patterns have different diffraction patterns, light will enter and exit lens 30 (FIG. 1) at different locations. Due to normal variations in lens surfaces, if light passes through different locations, lens aberrations will cause the light to diffract differently causing target displacement in the second layer 216.
The displacement error is a function of the mechanical placement capability of the system 20 and the projection lens 30 aberrations. The mechanical displacement is the same for both the pattern 120 and targets 110. However, lens aberrations affect the pattern 120 and targets 110 differently. In most cases, the lens induced error for the pattern 120 is smaller than the lens induced error in typical box-in-box targets 110. The lens error is more pronounced when different illumination shapes are used on two different layers. Since the aberrations change across the lens 30 the light is subject to different aberration patterns. Hence corrections based on typical box-in-box targets 110 or fine targets using the same pitch at both levels will induce displacement errors into the pattern 120.
There is a need and desire for a new method of designing feature dimensions, such as the pitch of second layer alignment targets, to minimize the impact of lens aberrations. Moreover there is a need to maximize the lens region overlap of light diffracted from two different illumination shapes. Furthermore, there is a need and desire for a new method for determining the pitch of a second layer targets based on the pitch and light diffraction patterns of a first layer target that minimizes displacement of the second layer targets by lens aberrations due to changes in illumination settings.
The invention relates to a method of determining a dimension for a semiconductor feature, in particular a second layer alignment target""s pitch, to minimize the impact of lens aberrations during optical projection. In an exemplary embodiment, the design method determines the pitch of a second layer fine pattern alignment target based on the light diffraction patterns of a first layer fine pattern alignment target. The first layer target is designed to have a pitch similar to that of a periodic feature of the integrated circuit, such as a capacitor. The second layer target is designed to have a pitch that minimizes displacement of the second layer target by optimizing the light diffraction patterns of the second layer target based on the first layer target.
The pitch of the second layer target is determined by several steps. First, projection lens locations of the light diffraction patterns created by the first layer target for a particular illumination setting are determined. Second, the projection lens locations of the 0th order light diffraction pattern for the second layer illumination settings are determined. Finally, the second layer target""s pitch is selected which optimizes overlap of the 1st order light diffraction patterns of the second layer target with that of the 0th order diffraction pattern of the first layer target. The more overlap between the respective diffraction patterns of the first and second layer targets, the more the displacement error caused by lens aberrations is reduced.